1. Field of the Invention
The present invention relates to a wire bonding method, a wire bonding apparatus, and a wire bonding control program for connecting a first bonding point and a second bonding point with a wire and, more specifically, to a wire bonding method, a wire bonding apparatus, and a wire bonding control program which are able to form a low loop.
2. Description of the Related Art
In a semiconductor device having a semiconductor chip integrated therein or in a substrate having the semiconductor chip mounted directly thereon, a lower-loop wire for connecting a pad and a lead of the semiconductor chip has been developed for reducing the thickness thereof.
In JP-A-2004-172477, United States Unexamined Patent Publication No. 2007-284416, U.S. Pat. No. 7,347,352, and JP-A-2005-019778, lower-loop wires are disclosed.
JP-A-2004-172477 includes a first step of connecting a wire to a first bonding point, a second step of controlling the loop by a reverse movement of a capillary such as moving the capillary upward or horizontally, a third step of bonding the wire on a portion near the top of a joint ball at the first bonding point, and a fourth step of connecting the wire to the second bonding point by feeding the wire while controlling the loop by moving the capillary upward or horizontally toward a second bonding point.
According to the disclosure in the US-A-2007-284416, a ball bond 14 is formed, then, a bonding tool is moved upward from the ball bond, and then a neck portion is formed on the upper portion of the ball bond with a reverse movement to move a capillary 10 away from a second bonding point to bend a wire. Subsequently, the capillary 10 is moved toward the second bonding point to position a periphery of a distal end of the bonding tool on the neck portion, and press the neck portion by the periphery of the distal end of the bonding tool to form a depression (recess) on the neck portion without joining the neck portion to the ball bond. Therefore, the strength of the neck portion is maintained, and the lower loop is achieved.
According to the disclosure of U.S. Pat. No. 7,347,352, a bump is formed on a first bonding site, a bent portion of the wire in an oval shape is formed, a wire is then bonded on top of the bump, and a wire loop is formed between the top of the bump and a second bonding site, so that a wire having a low loop height is formed.
According to the JP-A-2005-019778, after having formed a contact-bonded ball 11, a capillary 5 is moved upward, is moved in the direction opposite from a second bond point B to make a reverse movement, and then is moved slightly downward. Subsequently, the capillary 5 is moved upward, is moved toward the second bond point B to a position where a lower surface 5a of the capillary 5, which is the opposite side from the second bond point B, is positioned above the contact-bonded ball 11, and is moved downward to press a wire 4 against the contact-bonded ball 11 to form a contact-bonding wire 12 and form a strong bent portion on the wire right above the contact-bonded ball, so that a lower loop wire is achieved.
In recent years, in association with the improvement of functions of mobile phones, digital AV apparatuses, IC cards, and so on, compact, low profile, and high integration semiconductor chips are demanded to be mounted thereon. In order to accommodate such demands, it is desired to realize a lower profile in a high-integration IC package (semiconductor device) in the field of wire bonding. In order to achieve the low profile of the IC package, it is necessary to reduce the loop height, and the loop height which is currently required is not more than two times the wire diameter. Therefore, formation of a low loop which satisfies the loop height not more than two times the wire diameter is needed in the wire bonding and, for example, when the wire diameter of 25 μm is employed, the loop height is required to be 50 μm or lower.
Normally, in order to form a contact-bonded ball on a pad as a first bonding point and move the wire right above the contact-bonded ball laterally to form the wire horizontally, a height of at least two times or more of the wire diameter from the neck portion of the contact-ball is needed. Accordingly, the height of the wire from the neck portion of the contact-bonded ball exceeds 50 μm in the case of the wire having a wire diameter of 25 μm.
FIG. 12 is a drawing showing an example of the shape of the wire loop disclosed in JP-A-2004-172477. The wire loop shown in FIG. 12 is formed in such a manner that a wire 35 assumes a low loop between a pad 31 as the first bonding point on a semiconductor chip (IC chip) 30 and a lead 33 as the second bonding point of a lead frame 32.
As shown in FIG. 12, the wire loop disclosed in JP-A-2004-172477 is configured in such a manner that the ball is brought into press-contact with the first bonding point and then the wire 35 is bent by a reverse movement or the like, so that the wire 35 is pressed against the first bonding point again. Therefore, the wire loop in JP-A-2004-172477 has a wire turnover portion 50 between a contact-bonded ball 51 and the wire 35 formed by turning over the wire 35. A height LH1 of the loop in the shape of the wire loop shown in FIG. 12 is the sum of a ball thickness T1, a thickness T2 of the wire turnover portion 50, a height T3 of a protrusion of the loop, and a wire diameter T4.
However, in a loop forming method disclosed in JP-A-2004-172477, it is difficult to keep the sum of the ball thickness T1, the thickness T2 of the wire turnover portion 50, and the height T3 of a protrusion of the loop not to exceed 25 μm using the wire having a diameter of 25 μm, and hence when the wire diameter T4 is added thereto, it is difficult to keep the value of LH1 not to exceed 50 μm. Therefore, further reduction of the height of the loop is desired.
The neck portion integrated with the upper portion of the ball bond is formed in the United States Patent Publication No. 2007-284416, the wire bent portion having an oval shape is formed in U.S. Pat. No. 7,347,352, and the strong bent portion is formed on the wire right above the contact-bonded ball in JP-A-2005-019778. According to these documents, since the wire is further pressed against the upper portion of the contact-bonded ball, it is difficult to keep the loop height not to exceed two times the wire diameter as in JP-A-2004-172477.